The invention relates to naphtha hydrodesulfurization incorporating high temperature depressurization for mercaptan removal. More particularly, the invention relates to a naphtha hydrodesulfurization process, wherein the hot naphtha exiting the desulfurization reactor contains mercaptans, most of which are removed without olefin loss, by depressurizing the hot naphtha, thermally treating the hot naphtha, or some combination thereof. The desulfurized naphtha may be cooled and condensed to a liquid, separated from the gaseous H2S, stripped and sent to a mogas pool.
Motor gasoline (xe2x80x9cmogasxe2x80x9d) specifications are increasingly stringent, particularly with regard to sulfur content. Future regulations are expected to require that mogas contain no more than about 150 wppm of sulfur, as low as 30 wppm, or less. Such sulfur specifications may require the production of low sulfur blend stock for the mogas pool. The primary sulfur sources in the mogas pool are the blend stocks derived from fluidized catalytic cracking (xe2x80x9cFCCxe2x80x9d) of heavy oil, gas oil, and the like to form naphthas having a sulfur content in the range of 1000-7000 wppm depending upon crude quality and FCC operation. Conventional fixed bed hydrodesulfurization can reduce the sulfur level of FCC naphthas to very low levels, but the severe conditions of temperature, pressure and treat gas velocity results in significant octane number loss, due to extensive loss of olefins by saturation. Selective and severe hydrodesulfurization processes have been developed to avoid massive olefin saturation and octane loss. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,149,965; 4,243,519; 5,423,975; 5,525,211 and 5,906,730. However, in these and in other processes, in the hydrodesulfurization reactor the liberated H2S reacts with the retained olefins, to form mercaptan sulfur compounds. Depending on the amount of sulfur and olefins in the naphtha feed, the concentration of these reversion reaction product mercaptans typically exceeds fuel specifications for mercaptan sulfur and, in some cases, total sulfur.
For example, during naphtha hydrodesulfurization, the raw feed reacts with hydrogen in the presence of a hydrodesulfirization catalyst, at conditions of elevated temperature and pressure. This converts sulfur in organic sulfur-bearing compounds in the feed to H2S and forms a mixture of hot desulfurized feed and H2S. However, during the hydrodesulfurization, the H2S formed reacts with olefins in the feed to form mercaptans, irrespective of whether or not the feed being desulfurized contains mercaptans. These mercaptans formed as a consequence of the desulfurization are referred to as reversion mercaptans.
Generally it has been found that the mercaptans present in the hydrodesulfurized product have a higher carbon number than those found in the feed. These reversion mercaptans formed in the reactor, and which are present in the desulfurized product, typically comprise C4+ mercaptans. Others have proposed reducing the mercaptan and/or total sulfur of the hydrodesulfurized naphtha product by means such as 1) pretreating the feed to saturate diolefins, 2) extractive sweetening of the hydrotreated product, and 3) product sweetening with an oxidant, alkaline base and catalyst. However, none of these processes converts mercaptans.
It would therefore be desirable to convert mercaptans, particularly reversion mercaptans, to H2S and olefins in order to provide a further desulfurized naphtha without an undesirable loss in naphtha octane number.
The invention relates to high temperature depressurization for removing mercaptans including reversion mercaptans from hydrodesulfurized naphtha. More particularly, the invention relates to a naphtha desulfurization process, which comprises:
(a) hydrodesulfurizing a naphtha which contains olefins and sulfur in the form of organic sulfur compounds, to form a hydrodesulfurization effluent comprising a hot mixture of sulfur reduced naphtha, H2S and mercaptans, and then
(b) rapidly depressurizing at least a portion of the hydrodesulfurization effluent to destroy at least a portion of the mercaptans to form more H2S and a depressurized naphtha further reduced in sulfur.
In a preferred embodiment, the invention further comprises separating the H2S from the depressurized naphtha.
The depressurization is conducted at a high temperature, which is typically at least the temperature of the hydrodesulfurized naphtha effluent exiting the hydrodesulfurization reactor. The depressurization removes mercaptans without olefin loss due to saturation and even increases the olefin level in the desulfurized naphtha, to an amount slightly (e.g., less than 1 vol. %) higher than it would be without the depressurization.
In another embodiment, the invention relates to a naphtha desulfurization process, comprising:
(a) hydrodesulfurizing a naphtha, the naphtha containing olefins and sulfur in the form of organic sulfur compounds, to form a hydrodesulfurization effluent at an initial temperature, the effluent comprising a hot mixture of sulfur reduced naphtha, H2S and mercaptans; and then
(b) heating at least a portion of the hydrodesulfurization effluent to a temperature greater than the initial temperature at a substantially constant total pressure to destroy at least a portion of the mercaptans to form more H2S and a treated naphtha further reduced in sulfur.
In a preferred embodiment, the invention further comprises separating the H2S from the treated naphtha.
In one embodiment, at least a portion, and more preferably substantially all of the hydrodesulfuriztion effluent is in the vapor phase. In other words, the temperature of the hydrodesulfurization step is controlled so that it is above the dew point in the hydrodesulfurization reactor.
By depressurization is meant reducing the pressure to a level of no more than 50% of the pressure in the hydrodesulfurizing desulfurizing reactor, preferably no more than 25% and more preferably down to a level of no more than 10 % of that at the exit end of the reactor. In an absolute sense, the pressure after depressurization will be no more than 300 and preferably no more than 200 psig. If the hydrodesulfurization reactor is running at a fairly low pressure of 150 psig or less, the low pressure resulting from the depressurization will preferably be less than 25 psig and more preferably about atmospheric pressure. Preferably, the depressurization occurs in a depressurization reactor or vessel at a depressurization temperature. As the depressurization would be approximately adiabatic, a decrease in the depressurization temperature may be observed during the depressurization. Consequently, there will be an initial depressurization temperature at the start of depressurization and a lower, preferably slightly lower, final depressurization temperature at the conclusion of the depressurization step. For convenience, the depressurization temperature referred to herein in the initial depressurization temperature. While the depressurization temperature is preferably maintained at no less than the temperature of the hydrodesulfurized naphtha exiting the hydrodesulfurization reactor, it is more preferred that it be at least 10xc2x0 F. and still more preferably at least 25xc2x0 F. higher than that temperature. In one embodiment, the hydrodesulfurized naphtha is heated sufficiently so that the final depressurization temperature is above the dew point of the naphtha in the depressurization reactor. If more than one hydrodesulfurizing reactor is employed, it will typically be the effluent from the last reactor that is depressurized. The depressurization is conducted downstream of the hydrodesulfurization reactor or zone, typically in a separate vessel, and with or without the presence of a catalyst effective for increasing the rate of mercaptan decomposition. If a catalyst is present during the depressurization, the presence of hydrogen is preferred to avoid coking of the naphtha hydrocarbons. This process is particularly useful with naphtha feeds high in olefin and sulfur content and particularly with naphthas useful for gasoline, in which olefin retention is important for valuable octane.